The present invention relates to plasma diagnostics in integrated circuit processing.
Plasma etching is a fundamental and widely used technique in integrated circuit fabrication. In the simplest case a wafer is mounted on a ground electrode, and RF power is applied to another electrode which is parallel to the wafer, while a gas containing a species which contains a reactive element is flowed past the wafer. (For example, CF.sub.4 is a source of fluorine, and Cl.sub.2 is a source of chlorine.) The electrical excitation in the plasma volume will dissociate the source gasses to produce very reactive species.
Many variations of plasma etching can be distinguished (such as reactive ion etching, reactive sputter etching, ECR etching, and pre-excited etching), but all share the feature that RF power is used to create highly active chemical species in close proximity to the wafer (usually using a plasma at the wafer surface, to provide bombardment).
A basic requirement of plasma etching is endpoint detection. There are enough variations in thin film thickness and composition that it is highly desirable to know when an etching step is complete. This is usually done by an "endpoint detection" process, which senses the changes in the plasma chemistry when new layers are exposed. For example, during a metal etch the underlying dielectric will first be exposed when the etch has gone through the thinnest part of the metal, and some etch products from the dielectric may then appear in the plasma. As more of the dielectric is exposed, more of these etch products may appear in the plasma. When the pattern in the metal layer has been etched completely, the shift in the plasma chemistry will essentially stop.
Thus there is a great need for plasma sensors to detect such shifts in plasma chemistry. Plasma sensors are needed not only for endpoint detection, but also for hardware and process diagnostics and control of semiconductor manufacturing equipment.
The simplest plasma endpoint sensors simply look for a particular spectral line, usually at ultraviolet or short visible wavelengths. For example, when aluminum is exposed to a plasma etch process some aluminum will be present in the plasma, and the plasma will radiate at 261 nm; when aluminum is not present, radiation at this wavelength will be much weaker.
However, this conventional technique has some significant problems: one is degradation of the windows in the plasma reactor. A plasma generates very reactive chemical species, so the windows of the plasma reactor rapidly become cloudy due to etching or deposition on their interior surfaces. This cloudiness is particularly bad at shorter wavelengths (blue and ultraviolet). A second problem is the required filtering: the optical input at the wavelength being monitored is typically filtered with a time constant of the order of one second, which delays the eventual endpoint detection.
Some previous approaches to endpoint detection are shown in the following literature, all of which is hereby incorporated by reference: Griffiths and Degenkolb, 31 Appl. Spectrosc. 134 (1977); Angell and Oehrlein, "Grazing angle optical emission interferometry for end-point detection," 58 Appl. Phys. Lett. 240 (1981); Sternheim and voln Gelder, "A Laser Interferometer System to Monitor Dry Etching of Patterned Silicon," 130 J. Electrochem. Soc. 655 (1983); Kolodner et al., "End-point detection and etch-rate measurement during reactive-ion etching using fluorescent polymer films," 1 J. Vac. Sci. Tecnol. 501 (1983); Weiss, "Endpoint Monitors," Semiconductor International, September 1988, page 98.